Anionic-cationic polyion complexes comprising zwitterionic monomer component

ABSTRACT

A polyion complex (PIC) is formed of an anionic soluble polymer formed from ethylenically unsaturated monomers including at least one anionic or monomer and a cationic soluble polymer formed from ethylenically unsaturated monomer including at least one cationic monomer, in which the monomers used to form at lest one of the polymers comprise a switterionic monomer and in which the monomers used to form at least one of the polymers include non-ionic monomer, preferably C1-24 alkyl(meth)acrylate, in which the overall ratio of anionic groups to cationic groups in the PIC is in the range 1.5:1 to 1:1.5, and in which the polymers are combined in ratios to provide a PIC in which there are units derived from zwitterionic monomer in an amount in the range 1 to 70 mole % based on total monomer derived units in the PIC and there are units derived from non-ionic monomer in an amount in the range 0 to 60 mole % based on total monomer derived units in the PIC. The PIC is preferably swollen in water and is a flowable gel The zitterionic monomer is preferably 2-(methacyloyloxyethyl)-2′-(trimethylammoniumethyl)phosphate inner salt.

[0001] The present invention relates to polyion complexes, that isintimate blends of overall cationic polymers and overall anionicpolymers, at least one of which said polymers has pendant zwitterionicgroups to provide improved biocompatibility.

[0002] Physical gels are three-dimensional, disordered networks formedby associative forces that initiate noncovalent crosslinks. Mechanismsof interaction are numerous, including hydrogen bonding, hydrophobicinteractions, crystalline segment formation and ionic associationamongst others (Tanaka & Edwards, Macromolecules, 25, 1516, 1992).

[0003] In contrast to chemical gels that have defined point crosslinks,physical gels have so-called junctions zones where linear segments ofthe polymer chain form ordered structures. The nature and number ofthese zones determine the differences between gels.

[0004] It is well known in the literature that there are water-solublepolymers that contain complexing groups, whether neutral or charged,that can form gels by ionic association. This is most commonly achievedby the presence of sufficient inorganic metal salts under theappropriate conditions. The bonding chemistry between the metal ions isspecific, each forming a gel with different polymers under specificconditions of pH, ionic strength and concentration of the polymer.

[0005] Another method of obtaining a gel by complexation of relevance tothis invention, is by the formation of an interpolymer complex. As itsname suggests, this is a process by which two distinct polymer entitiesinteract to form a complex. If complexation is achieved by theinteraction of oppositely charged ionic groups within the two polymers,the system is termed a polyion or polyelectrolyte complex (Michaels,Indust. & Eng. Chemistry, 57 32, 1965). If the interaction is between astrongly acidic polyanion and a strongly basic polycation, coulombicforces at the polyion sites results in the release of a microanion andmicrocation (the counterions of the original polyelectroytes), which arethen free to diffuse into the body ofthe solvent. The reaction willpropagate rapidly from site to site, releasing the microions providingthe entropy increase upon their liberation is not outweighed by theentropy decrease upon collapse and condensation ofthe polyion pair.

[0006] The polyion complexes have the potential to be solubilised internary solvent systems consisting of water, a water-soluble organicsolvent like acetone and a strongly ionised simple electrolyte such asNaBr. This allows fabrication into many forms including fibres, filmsand coatings. This, together with their reported inherentnon-thrombogenic nature, has made these materials interesting asbiomaterials (Ratner & Hoffman, ACS Symposium Series 71 ed. J. D.Andrade, ACS, Washington D.C., 1976, p1).

[0007] The use of polyion complexes in medical applications has beensuggested for many years. Indeed, Michaels made reference to the use ofsuch complex solutions for potting or encapsulating aneurysms,commenting that the materials were reasonably well tolerated by thetissue. Ioplex 101 (a complex poly(triethyl-(3 & 4)-vinylphenylammoniumbromide) and poly(sodium vinyl benzenesulphonate)) has been examinedintensively for biomedical usage (Vogel el al. J. MacromoL Sci., Chem.,4, 675, 1970; Marshall et al., J. Biomed Mater. Res.,4, 357,1970; Brucket al., Ann. N.Y. Acad Sci., 283 332, 1977). Analogues of this systemhave been studied to determine the effect of charge and structure on thecomplex and their behaviour towards blood platelets (Kataoka el al.,Makromol. Chem., 179 1121, 1978 & 181, 1363, 1980) and have been used asencapsulating agents in the development of artificial liver supportsystems (Kataoka et al., Jinko Zoki (Artificial Organs), 8 296, 1979).

[0008] Nakabayashi et al have previously described the use of polyioncomplexes of polymers having <zwitterionic pendant groups for theselective adhesion of platelets (J. Biomed. Mater. Res., 28(11), 1347,1994 by Ishihara et al. Adv. Biomat. Biomed. Eng. Drug Delivery Syst.(1995) 227-228 by Ishihara, K. et al., and Japanese PatentJP-A-7-238124). Their invention claims specifically the use of a ternarypolymer system consisting of 2-methacroyoyloxyethyl phosphorylcholine(MPC), butyl methacrylate (BMA) and sulfur propyl methacrylate (SPM) ortrimethyl anmmonium propyl methacrylate (TPM). Further to this, theydefine the compositions in which the MPC:BMA molar ratio is between2:98 - 50:50, and the ratio of these two components to the ionic monomer(SPM or TPM) is between 98:2-80:20. These systems seem to have beendesigned to produce coatings with weak ionic interactions that havefavourable properties in terms of platelet binding and activation. Theanionic and cationic polymers are water insoluble, alcohol soluble. Thepolyion complexes described in these references are tested as coatingson glass beads and one of the products is said to be under test for useto encapsulate activated charcoal used for an artificial liver supportsystem. Coatings of the PIC's are produced by mixing preformed solutionsat 10% solids concentrations of the terpolymers each in ethanol, dippingthe substrate to be coated in the solution and allowing the alcohol toevaporate from the film of coating composition.

[0009] JP-A-08-165491 (1996) describes complexes formed of a polymerhaving an overall cationic charge and which further includes pendanthydrophobic groups and pendant carboxybetaine groups, with an anionicsurfactant such as α-olefin sulphonates and fatty acid soaps. Thecomplexes are flexible solids and are for use with detergent components.

[0010] JP-A-10-245325 (1998) describeshair setting compositionscomprising a cationic polymer having pendant hydrophobic groups, and apolymer having pendant carboxybetaine groups and pendant hydrophobicgroups.

[0011] According to the invention there is provided a new method inwhich a solution of an anionic polymer having an overall anionic chargeand a cationic polymer having an overall cationic charge together in asolvent system comprising a first solvent and an inorganic salt, insolution, is gelled by contact with water, whereby the ions of theinorganic salts become dissociated from the polymer and extracted fromthe gel formed by electrostatic attraction between polymer boundcationic groups and polymer bound anionic groups, and is characterisedin that at least one of the cationic and anionic polymers compriseszwitterionic groups.

[0012] The method of the invention thus involves a transformation of thepolymer from being in a mobile solution or suspension form to being agel. The method generally involves collapsing of the gel, that is thegel has a lower volume than the starting volume of the solution in thesolvent system.

[0013] In the method, the solvent system generally comprises an organicsolvent. Preferably the organic solvent is water-miscible. Mostpreferably the solvent system comprises at least two solvents, in whichthe second solvent is water.

[0014] Examples of suitable organic solvents for use in the solventsystems are alcohols, ethers, esters and, most preferably, ketones. Mostpreferably the solvent is a ketone such as acetone.

[0015] For a solvent system comprising two solvents, these are generallyused in a ratio in the range 5:1 to 1:5. Preferably the range of organicsolvent water is in the range 2:1 to 1:5, preferably 1:1 to 1:4.

[0016] The inorganic salts should be soluble in the solvent system.Where the solvent system contains water, therefore, the salt should bewater-soluble, for instance at a concentration of at least 10% byweight. Preferably the salt is soluble in a concentration of at least20% by weight.

[0017] Preferably the salt comprises a single, preferably monovalentmetal salt. Di- or higher valent metal salts may cause prematurecoagulation or gelation. Likewise the anion of the salt is preferably asinly charged anion, preferably of a strong acid, most preferably otherthan an oxyanion although some oxyanions may be useful. Preferably theanion is a halide. The salt is preferably a halide of an alkali metal.The alkali metal is lithium, potassium, or, preferably, sodium. Thehalide is suitably chloride, bromide or iodide.

[0018] The salt is preferably present in the solvent system in an amountof at least 2% by weight, preferably at least 5% by weight, for instanceup to 20%/ by weight. Preferably the salt is present in an amount in therange 5 to 15% by weight.

[0019] In the method of the invention, the polymer bound cationic andanionic groups may comprise charged atoms in the backbone of themolecule. Cationic groups formed in the backbone of the polymer may, forinstance, be secondary, tertiary or quaternary ammonium groups.Preferably, however, the cationic and anionic groups ofthe polymers arependant groups. Likewise the zwitterionic monomer is preferably apendant group.

[0020] The individual polymers used in the method of the invention arepreferably water-soluble, for instance produce a clear solution at aconcentration of at least 1%, more preferably at least 5%, by weight.

[0021] The mixed solution of the two polymers in the solvent system maybe generated by mixing together preformed solutions of the individualpolymers in portions of the solvent system, or components thereof. It isgenerally preferred that all of the components are present incombination when the polymers first contact one another, in order toavoid premature gelling. Mixing procedures generally involve adequatestirring and temperatures which provide the desired solubility.

[0022] The individual polymers may be of low or high molecular weight.Preferably the molecular weight is low enough for the solution of theindividual monomer in a single solvent to be mobile and of lowviscosity, to optimise handling. It is preferred that the inherentviscosity of the polymer solutions according to the test providedhereinafter is in the range 5 to 500 mPa.s, more preferably in the range10 to 300 mPa.s, for instance in the range 20 to 150 mPa.s.

[0023] In the method of the invention it is preferred that both of thepolymers have zwitterionic groups, preferably zwitterionic pendantgroups. It is preferred for the zwitterionic group to comprise amonovalent anion and monovalent cation. Where the zwitterion comprisesan excess of anion over cation or vice versa, the zwitterionic group mayfinction as both polymer bound anion or cation, as the case may be, andzwitterion.

[0024] In the method, it is preferred that approximately equivalentlevels of anionic and cationic groups are present so that the anioniccharges and cationic charges are balanced. It is preferred for thepolymer mixture to have substantially no overall charge. It is believedthat this characteristic optimises biocompatibility, especiallyhaemocompatibility. The worked examples described below show that thegels have low protein adsorption properties.

[0025] In the method of the present invention water is contacted withthe solution of the mixed polymers in the solvent system by any suitablemeans. For instance the interface of a body of the solution with watermay be provided at the surface of a coating on a substrate, or thesolution may be restrained in a mould providing means for contacting thesolution in the mould with water. Liquid excluded from the gel, uponcollapse, for instance, may be removed by evaporation or by drainingfrom the gel. The water may be contacted with the solution by spraying,flowing or dipping.

[0026] Some of the PIC's formed by the new phase change, gelificationprocedure, are novel in themselves. Thus PIC's formed from combinationsof certain selected polymers have not been disclosed in the prior art.It may be possible to make the PIC's by alternative processes, such asby depositing them from a mixed solvent system in which they aresoluble, followed be evaporation of the solvent, that is using thegeneral procedure described by Ishihara et al., op.cit.

[0027] Preferred zwitterions, cations and anions and monomers from whichpolymers used in the method of the invention are described below.

[0028] A new polyion complex according to the present invention isformed from a cationic polymer having an overall cationic charge and ananionic polymer having an overall anionic charge, in which the anionicpolymer is obtainable by polymerising ethylenically unsaturated monomerscomprising:

[0029] a) 5 to 100 mole % anionic monomer having an anionic oranionisable group;

[0030] b) 0 to 85 mole % zwitterionic monomer having a zwitterionicgroup; and

[0031] c) 0 to 80 mole % nonionic monomer;

[0032] and in which the cationic polymer is obtainable by polymerisingethylenically unsaturated monomers including

[0033] d) 5 to 100 cationic monomer having a cationic or cationisablegroup;

[0034] e) 0 to 85 mole % zwitterionic monomer having a pendantzwitterionic group; and

[0035] f) 0 to 80 mole % non ionic monomer;

[0036] in which the total units in the polyion complex derivable fromnonionic monomer c and f is in the range 0 to 60 mole %, the total mole% of units in the PIC derivable from zwitterionic monomer is in therange 1 to 70 mole %, and the ratio of moles of anionic or anionisablegroups in the anionic polymer to the moles of cationic or cationisablegroups in the cationic polymer is in the range 1.5:1 to 1:1. 5.

[0037] According to a fuirther aspect of the present invention a newpolyion complex is formed from a cationic polymer having an overallcationic charge and an anionic polymer having an overall anionic charge,in which the anionic polymer is water soluble and is obtainable bypolymerising monomers including

[0038] a) 5 to 100 % anionic monomer having an anionic or anionisablegroup;

[0039] b) 0 to 85 mole % zwitterionic monomer having pendantzwitterionic group; and

[0040] c) 0 to 60 mole % non ionic monomer;

[0041] and in which the cationic polymer is water soluble and isobtainable by polymerising ethylenically unsaturated monomers including

[0042] d) 5 to 100 mole % cationic monomer having a cationic orcationisable group;

[0043] e) 0 to 85 mole % zwitterionic monomer having a zwitterionicpendant group; and

[0044] f) 0 to 60 mole % nonionic monomer;

[0045] in which the total moles of units in the PIC derivable fromzwitterionic monomers in the range 1 to 70 mole %, and in which theratio of equivalents of anionic groups in anionic polymer to equivalentsof cationic groups in cationic polymer is in the range 1.5:1 to 1:1.5.

[0046] In all aspects of the invention, the total level of unitsderivable from nonionic monomer in the PIC is preferably at least 5 mole%.

[0047] In all aspects of the invention, the anionic polymer preferablydoes not include units derivable from cationic monomer and the cationicpolymer preferably does not include units derivable from anionicmonomer.

[0048] The components of the PIC, in terms of the cationic and anionicpolymers and the monomers from which each polymer is made, are generallyselected such that an aqueous gel of the novel PIC product of the novelprocess flows under imposition of the force rendering it capable ofbeing pumped. For PIC's formed from cationic and anionic polymers havingrelatively high proportions of cationic and anionic groups,respectively, the desired properties are achievable by using relativelyhigh proportions of zwitterionic monomer. For instance the total unitsderivable from zwitterionic monomer in the PIC is preferably at least 30mole %, generally less than 50 mole %. Where the total moles of ionicmonomer in the PIC is less than 30, for instance in the range 10 to 30mole %, then the level ofunits in the PIC derivable from zwitterionicmonomer is preferably in the range 15 to 70 mole %. Where the totalmoles of ionic monomer in the PIC is in the range 5 to 10 mole %, thenthe level of units derivable from zwitterionic monomer in the PIC ispreferably in the range 70 to 30 mole %.

[0049] The ratio of equivalents of anionic groups in anionic polymer toequivalents of cationic groups in cationic polymer (not includingneutralised cation/anion pairs of a zwitterionic group) is preferably inthe range 1.25:1 to 1:1.25, more preferably in the range 1.1:1 to 1:1.1, preferably about 1:1. Preferably therefore the PIC have no overallcharge.

[0050] The PIC should generally be water-insoluble, but water-swellable.The PIC's may absorb more than their own weight of water, often morethan twice their own weight for instance up to 10 times their ownweight.

[0051] The rheological properties of the PIC, for instance swollen bywater, may be determined by using a variable torque oscillation test ina suitable rheometer. Such a device can determine the elasticity modulusand the viscous modulus. The present invention is directed in particularto PIC's which, when swollen in water, and subjected to the test as setout in the following paragraph have values of G′ (elasticity modulus)and G″W (viscous modulus) of G′ in the range 1 to 1000 and G″ in therange 5 to 1000. Generally the test is conducted when the gels are fullyswollen in water.

[0052] The viscoelastic properties are determined using a variabletorque oscillation test (80 mN.m) using a TA instrument CSL-100rheometer fitted with 6cm 2° cone at 37° C.

[0053] The zwitterionic pendant group of the polymer used in theinvention may have an overall charge, for instance by having a divalentcentre of anionic charge and monovalent centre of cationic charge orvice-versa or by having two centres of cationic charge and one centre ofanionic charge or vice-versa. Preferably, however, the zwitterion has nooverall charge and most preferably has a centre of monovalent cationiccharge and a centre of monovalent anionic charge.

[0054] Preferably the centre of cationic charge in the zwitterionicgroup is permanent, that is it is preferably a quaternary ammonium orphosphonium or tertiary sulphonium group. Preferably the anion ispermanent, that is it is substantially completely ionised at in vivopH's, for instance at pH's in the range 5 to 8. It is preferably aphosphate, phosphonate, sulphate or sulphonate anion.

[0055] The zwitterionic group may be a betaine group (ie in which thecation is closer to the backbone), for instance a sulpho-, carboxy- orphospho-betaine. A betaine group should have no overall charge and ispreferably therefore a carboxy- or sulpho-betaine. If it is aphosphobetaine the phosphate terminal group must be a diester, i.e., beesterified with an alcohol. Such groups may be represented by thegeneral formula I

—X²—R²—N^(⊕)(R³)₂—R⁴—V^(⊖)  I

[0056] in which X² is a valence bond, —O—, —S— or —NH—, preferably —O—;

[0057] V is a carboxylate, sulphonate or phosphate diester(monovalentlycharged) anion;

[0058] R² is a valence bond (together with X²) or alkanedyl—C(O)alkanediyl— or —C(O)NHalkanediyl preferably alkanediyl andpreferably containing from 1 to 6 carbon atoms in the alkanediyl chain;

[0059] the groups R³ are the same or different and each is hydrogen oralkyl of 1 to 4 carbon atoms or the groups R³ together with the nitrogento which they are attached form a heterocyclic ring of 5 to 7 atoms; and

[0060] R⁴ is alkanediyl of 1 to 20, preferably 1 to 10, more preferably1 to 6 carbon atoms.

[0061] One preferred sulphobetaine monomer has the formula III

[0062] where the groups R⁵ are the same or different and each ishydrogen or C₁₋₄ alkyl and n is from2to 4.

[0063] Preferably the groups R⁵ are the same. It is also preferable thatat least one ofthe groups R⁵ is methyl, and more preferable that thegroups R⁵ are both methyl.

[0064] Preferably n is 2 or 3, more preferably 3.

[0065] Alternatively the zwitterionic group may be an amino acid moietyin which the alpha carbon atom (to which an amine group and thecarboxylic acid group are attached) is joined through a linker group tothe backbone of polymer A. Such groups may be represented by the generalformula III

[0066] in which X³ is a valence bond, —O—, —S— or —NH—, preferably —O—,R⁶ is a valence bond (optionally together with X³) or alkanediyl,—C(O)alkanediyl— or —C(O)NHalkanediyl, preferably alkanediyl andpreferably containing from 1 to 6 carbon atoms; and

[0067] the groups R⁷ are the same or different and each is hydrogen oralkyl of 1 to 4 carbon atoms, preferably methyl, or two of the groupsR⁷, together with the nitrogen to which they are attached, form aheterocyclic ring of from 5 to 7 atoms, or the three group R⁷ togetherwith the nitrogen atom to which they are attached form a fused ringstructure containing from 5 to 7 atoms in each ring.

[0068] Preferably the zwitterion has the formula IV

[0069] in which the moieties X⁴ and X⁵, which are the same or different,are —O—, —S—, —NH— or a valence bond, preferably —O—, and

[0070] W⁺ is a group comprising an anrnonium, phosphonium or sulphoniumcationic group and a group linking the anionic and cationic moietieswhich is preferably a C₁₋₁₂—alkylene group.

[0071] Preferably W contains as cationic group an ammonium group, morepreferably a quatemary ammomum group.

[0072] The group W⁺ may for example be a group of formula

—W¹-N^(+R) ⁸ ₃, —W¹—P⁺R⁹ ₃, —W¹—S⁺R⁹ ₂ or —W¹—Het⁺ in which:

[0073] W¹ is alkanediyl of 1 or more, preferably 2-6, carbon atomsoptionally containing one or more ethylenically unsaturated double ortriple bonds, disubstituted-aryl, alkylene aryl, aryl alkylene, oralkylene aryl alkylene, disubstituted cycloalkyl, alkylene cycloalkylcycloalkyl alkylene or alkylene cycloalkyl alkylene, which group W¹optionally contains one or more fluorine substituents and/or one or morefunctional groups; and

[0074] either the groups R⁸ are the same or different and each ishydrogen or alkyl of 1 to 4 carbon atoms, preferably methyl, or aryl,such as phenyl, or two of the groups R⁸ together with the nitrogen atomto which they are attached form a heterocyclic ring containing from 5 to7 atoms or the three groups R⁸ together with the nitrogen atom to whichthey are attached form a fused ring structure containing from 5 to 7atoms in each ring, and optionally one or more of the groups R⁸ issubstituted by a hydrophilic functional group, and

[0075] the groups R⁹ are the same or different and each is Rs or a groupOR⁸, where R⁸ is as defined above; or

[0076] Het is an aromatic nitrogen-, phosphorus- or sulphur-, preferablynitrogen-, containing ring, for example pyridine.

[0077] Preferably W¹ is a straight-chain alkanediyl group, mostpreferably 1,2-thanediyl.

[0078] Preferred groups of the formula IV are groups of formula V:

[0079] where the groups R¹⁰ are the same or different and each ishydrogen or C₁₋₄ alkyl, and m is from 1 to 4.

[0080] Preferably the groups R¹⁰ are the same. It is also preferablethat at least one of the groups R¹⁰ is methyl, and more preferable thatthe groups R¹⁰ are all methyl.

[0081] Preferably m is 2 or 3, more preferably 2.

[0082] Alternatively the ammonium phosphate ester group V may bereplaced by a glycerol derivative of the formula VB, VC or VD defined inour earlier publication no WO-A-93/01221.

[0083] The zwitterionic monomer preferably has the formula VI

YBX   VI

[0084] wherein

[0085] B is a straight or branched alkanediyl, oralkanediyloxaalkanediyl or alkanediyl oligo (oxaalkanediyl) chainoptionally containing one or more fluorine atoms up to and includingperfluorinated chains or, if X or Y contains a terminal carbon atombonded to B, a valence bond;

[0086] X is the zwitterionic group; and

[0087] Y is an ethylenically unsaturated polymerisable group selectedfrom

[0088] CH₂═C(R)—CH₂—O—, CH₂═C(R)—CH₂OC(O)—, CH₂═C(R)OC(O)—, CH₂═C(R)O—,CH₂═C(R)CH₂OC(O)N(R¹¹)—, R¹²OOCCR═CRC(O)—O—, RCH=CHC(O)O—,RCH═C(COOR¹²)CH₂—C(O)O—,

[0089] wherein:

[0090] R is hydrogen or a C₁-C₄ alkyl group;

[0091] R¹¹ is hydrogen or a C₁-C₄ alkyl group or R¹¹ is —B—X where B andX are as defined above; and

[0092] R¹² is hydrogen or a C₁₋₄ alkyl group or BX where B and X are asdefined above;

[0093] A is —O— or —NR¹¹;

[0094] K is a group —(CH₂)_(p)OC(O)—, —(CH₂)_(p)C(O)O—,—(CH₂)_(p)OC(O)O—, —(CH₂)_(p)NR¹³—, —(CH₂)_(p)NR¹³C(O)—,—(CH)_(p)C(O)NR1 ¹³—, —(CHD)_(p)NR¹³C(O)O—, —(CH₂ _(p)OC(O)NR¹³—,—(CH₂)_(p)NR¹³C(O)NR¹³— (in which the groups R¹³ are the same ordifferent), —(CH₂)_(p)O—, —(CH₂)_(p)SO₃ —, or, optionally in combinationwith B, a valence bond and p is from 1 to 12 and R¹³ is hydrogen or aC₁-C₄ alkyl group.

[0095] Preferably Y is a group CH₂═C(R)COA—, in which R is H or methyl,preferably methyl, and in which A is preferably O.

[0096] B is preferably an alkanediyl group of 1 to 12, preferably 2 to 6carbon atoms, most preferably group (CH₂)_(q) in which q is 2 to 6.

[0097] Each of the cationic and anionic monomers may be represented bythe formula VII

Y¹B¹Q   VII

[0098] in which Y¹ is selected from the same groups as Y

[0099] B¹ is selected from the same groups as B; and

[0100] Q is an ionic or ionisable group. Q may be a cationic group Q¹ oran anionic group Q².

[0101] In some embodiments of the present invention, a polycationicpolymer will have permanently cationic pendant groups. These may bequaternary ammonium or phosphonium groups. In other embodiments, thecationic group may not be a permanent cation. It may be a weak or astrong base. For instance it may be selected so as to provide pHsensitivity whereby the degree of attraction between the two firstpolymers may be controlled by the pH.

[0102] Likewise, the anion may be the anion of a weak or strong acid,selected so as to be pH sensitive or insensitive within a predeterminedpH range, as desired.

[0103] A suitable cationic group Q¹ is preferably a group N⁺R¹ ₃,P^(+l R) ¹ ₃ or S⁺R¹ ₂

[0104] in which the groups R¹ are the same or different and are eachhydrogen, C₁₋₄-alkyl or aryl (preferably phenyl) or two of the groups R¹together with the heteroatom to which they are attached from a saturatedor unsaturated heterocyclic ring containing from 5 to 7 atoms.Preferably the cationic group is permanently cationic, that is each R¹is other than hydrogen. Preferably the cationic group is N⁺R¹ ₃ in whicheach R¹ is C₁₋₄-alkyl, preferably methyl.

[0105] Suitable anionic groups Q² are carboxylate, carbonate,sulphonate, sulphate, phosphonate or phosphate. Preferably the anionicgroup is monovalent. A sulphonate group is particularly convenient.

[0106] Another suitable type of cationic monomer copolymerisable withethylenically unsaturated monomers is diallyl dialkyl ammonium halide,for instance diallyl dimethyl ammonium chloride.

[0107] Nonionic monomer included in either or both of the cationic andanionic polymer is selected so as to confer desired solubility,hydrophilicity or hydrophobicity properties, viscosity properties on theindividual polymers and on the PIC. Hydrophobic groups may provide interor intra molecular interactions with hydrophobic groups, or withsubstrates or biological compounds in contact with the PIC in use.

[0108] Preferably a nonionic monomer has the general formula VIII

Y² R¹⁴   VIII

[0109] in which Y² is selected from the same groups as Y; and

[0110] R¹⁴ is a nonionic organic group which is an optionallysubstituted C₁₋₂₄-alkyl or -alkenyl group. Optional substituents in thealkyl or alkenyl group are hydroxyl groups, halogen atoms, alkoxy andoligo-alkoxy groups, in which the alkoxy groups have 1-6, preferably 2or 3 carbon atoms, aryl groups, preferably optionally substituted phenylgroups (optional substituents in a phenyl group being hydroxyl groups,halogen atoms or alkyl groups), acyl groups, especially C₁₋₆-alkanoylgroups, acyloxy groups, especially C₁₋₆-alkanoyloxy groups or acylaminogroups, especially C₁₋₆-alkanoyl amino, in any of which alkanoyl andacyl groups there may be substituents selected from halogen atoms andhydroxyl groups. Preferred groups R¹⁴ are C₁₋₂₄ unsubstituted alkyl,more preferably. C₄₋₁₈ alky.

[0111] Where the PIC is used as a gel swollen with a liquid, the liquidmay be derived from, that is consist of, solvents from which the anionicand cationic polymers are presented in a method of forming the PIC bymixing two preformed solutions. Simce each ofthe polymers is preferablywater-soluble, and since it may often be convenient for the PIC to beswollen in water, preferably both the anionic and cationic polymers aredissolved in an aqueous solvent.

[0112] In the method of the invention by admixing two preformedsolutions, each ofthe solutions preferably contains polymer in amountsin the range 0.1 to 50% by weight, preferably in the range 1 to 50%, forinstance in the range 10 to 25% by weight.

[0113] Preferably the water-swellability of the PIC is such that the PICwill absorb deionised water in an amount of 10 to 1000% based on theweight of polymer, preferably in the range 50 to 500%.

[0114] The polymer solutions are mixed in the method of the invention soas to allow intimate contact between the counterionically chargedpolymers. It is preferable that, after the solutions have been mixed,that the mixture is allowed to rest for a period to develop gelproperties.

[0115] The gel of the PIC swollen in a liquid may be used immediatelywithout further processing. Alternatively it may be desirable to recoverthe PIC from the liquid vehicle and re-gel the PIC in an alternativesolvent, or in the same type of solvent, optionally after rinsing in thesame or other solvent, for instance to extract salts formed fromcounterions ofthe anionic and cationic pendant groups in the respectivestarting polymers (the microions).

[0116] Whilst the PIC is generally water-insoluble, it may be possibleto dissolve, redissolve or disperse the PIC in a non-aqueous solventsuch as an alcohol or ether solvent, or in a solvent system such as isused in the new method of the invention. A solution of the PIC in such asolvent may be useful as a coating composition, for coating substratesto improve their biocompatibility.

[0117] The method of the invention may suitably be carried out in situto provide a gel product in a desired location, such as in contact witha biological liquid or with tissue.

[0118] The PIC's of the present invention are believed to have desirablebiocompatibility and are useful in environments where PIC's havepreviously been used such as in compositions to be used in contact withblood, for instance in embolising blood vessels.

[0119] Other potential uses of the PIC's are in in situ coating of theinternal surfaces of blood vessels, known as endoluminal gel-paving, asdescribed in WO-A-9112846 and WO-A-9001969, filling of wound cavities,as fillers for various therapeutic and cosmetic purposes, e.g. for usefollowing tumour excision, for use to improve muscle control, e.g. ofsphincter muscles to control incontinence, as a supplement to synovialfluid, a filler for use in the treatment of patent ductus arteriosis,etc.

[0120] The PIC's may be used in products in which a pharmaceuticallyactive agent or a diagnostic agent is incorporated. For instance the PICmay be a drug delivery depot from which pharmaceutically activeingredient may be delivered over time systemically or locally in apatient. A diagnostic agent may, for instance, be a radiopaquecomponent, such as dispersed particulate radiopaque material (bariumsulphate, for instance), or may be a solid device having a particularshape, such as a coil, filament, wire or thread of a metal. A radiopaquematerial may allow visualisation of the PIC in situ and the surroundingenvironment.

[0121] In the drawings

[0122]FIG. 1 is a phase diagram for the formation of polyion complexesfrom systems based on Mpc_(x)Bma_(y)Tem_(z) and Mpc_(x)Bma_(y)Spm_(z)(see below for abbreviations);

[0123]FIG. 2 is a generalised diagram for the formation of polyioncomplexes; and

[0124]FIG. 3 is a phase diagram for the formation of polyion complexesfrom systems based on Mpc_(x)Gma_(y)Tem_(z) and Mpc_(x)Bma_(y)Spm_(z).

[0125] The invention is illustrated further in the accompanyingexamples. In these examples, the following standard methods are used:

[0126] Inherent Viscosity

[0127] 20% w/v solutions were made of each polymer using deionisedwater. The solution was subjected to a flow test (shear rate 1-1000so⁻¹) using a TA Instruments CSL²-100 Rheometer fitted with a 6 cm 2°cone at a temperature of 37° C. From the 3 0 resulting viscosity vs.shear rate trace, the viscosity (Pa.s) ofthe solution was determined bytaking the value at 200 s⁻¹.

[0128] Fibrinogen Adsorption

[0129] This test is carried out substantially as described inWO-A-93/01221.

[0130] Bicinchoninic Acid Protein Assay

[0131] Assessment of protein adsorption was carried out using theMicro-Bicinchoninic Acid (m-BCA) Protein Assay (Pierce & Warriner kit),which relies on the colourimetric detection of a Cu(I) complex with BCAproduced upon protein reduction of Cu(H) to Cu(I). Coated and uncoatedPET strips were prepared as described for the immunoassay, except thatin this case they were cut in half and assayed as two 9×15 mm strips.Samples were incubated in 4 ml of 0.5 mgml⁻¹ of fibrinogen solution for10 minutes at room temperature. Sample blanks of uncoated PET stripswere incubated in 4 ml of PBS in the same manner. Both samples andblanks were washed in a DiaCent 2000 cell washer and then transferred toclean tubes and incubated with 100 μg 1 PBS and 1 ml m-BCA workingreagent at 60° C. A Bovine Serum Albumin (BSA) standard curve wasconstructed so as to give the required amount of protein in 100 μlsolution. Standards were incubated with lml of working reagent as above.The absorbance of a 300 μl aliquot of the sample was measured in amicroplate reader at 562 nm.

[0132] Abbreviations Used: Monomer Code Chemical Name MpcMethacryloxyethyl phosphorylcholine (2-methacryloyloxyethyl-2′-trimethylammoniumethyl phosphate inner salt) BmaButyl methacrylate (hydrophobic diluent) Tem 2-trimethylammonium ethylmethacrylate chloride salt Spm 3-methacryloyloxypropylsulphonatepotassium salt EtOH ethanol TFE 2,2,2-trifluoroethanol THFtetrahydrofuran MeOH methanol DI Water deionised water DCMdichloromethane PET polyethyleneterephthalate PBS phosphate bufferedsaline

EXAMPLE 1 Generic Method for the Preparation of PC-Containing Polyions.

[0133] The polymers were developed using free radical solutionpolymerisation techniques following the standard method outlined below.2-(methacryloyloxyethyl)-2′-(trimethyl-ammoniumethyl) phosphate, innersalt (Mpc) was prepared according to the method described previouslyWO-A-95/14702. Bma, Spm and Bma are all commercially available.

[0134] A triple-necked round bottom flask (500 ml) was equipped with aDavis condenser, a nitrogen inlet and a thermometer. The condenser wastopped with a calcium chloride guard tube, and a magnetic follower wasadded to the flask. The reaction system then purged using nitrogen gas.

[0135] The required amount of Mpc was weighed and then stirred in asuitable reaction solvent until dissolved. To this was added theappropriate amounts of the other comonomers (ionic monomer andhydrophobic diluent if required). The initiator type and level waschosen depending upon the reaction solvent employed.

[0136] The solutions were then filtered under vacuum using a Buchnerfimnel, into the reaction vessel. The solution was degassed using aconstant flow of nitrogen for a period of twenty minutes, after whichtime the nitrogen flow rate was reduced and the temperature increased tosuitable level dictated by the reaction solvent in use. Thepolymerisation was carried out under an atmosphere of nitrogen, andmaintained at temperature for a period between 1640 hours.

[0137] When the polymerisation had finished the heat source was removedand the solution was allowed to cool to room temperature. In the casewhere a volatile reaction solvent or solvent mixture had been used, thesolvent was removed using rotary evaporation techniques until the pointat which the polymer began to foam. This foam was then furtherredissolved in a suitable solvent/non-solvent combination (typically 9:1DCM:MeOH) and precipitated by dropwise addition into a non solvent,typically acetone (1000 ml) with constant stirring. The precipitate wasthen collected using vacuum filtration under a blanket of nitrogen anddried at 50° C. in vacuo for 16 hours.

[0138] In the case where water was used as the reaction solvent, thesolution was allowed to cool and the polymer purified by ultrafiltrationto remove low molecular weight species. The polymer could be isolated byfreeze drying for subsequent analysis.

[0139] Once isolated, the individual polymers were subjected to NMR andelemental analysis to confirm the structure.

[0140] Table 1 summarises the preparative details for a selected rangeof polyion compounds and Table 2 the isolation details for thosepolymers. Table 3 provides some characterisation for the polymers interms of 1H NMR. Elemental analysis was acceptable compared totheoretical values for most cases (within 10%,error as expected forpolymers); table 4 however, summarises the key elemental data,concentrating on phosphorus:nitrogen and phosphorus:sulphur ratios inorder to determine extent of Tem and Spm incorporation in the respectivepolycations and anions. This can subsequently be used to better definethe final polymer composition versus the feed monomer ratios (as shownin table 1 to 3). The inherent viscosity of 20% w/v aqueous solutions ofthe polyions was obtained by rheometry, as an approximate indicator ofmolecular weight, and is reported in Table 5.

EXAMPLE 2 Formation of Polvion Complexes (PIC's) by Mixture of AqueousSolutions of PC-Containine Polvelectrolvtes.

[0141] Table 6 summarises some of the observations made upon mixing 20%w/v aqueous solutions of various polyions produced in Example 1 (theratios are for the monomer in the polymerisation mixture rather than inthe polymer by analysis).

[0142] 0.5 g of each polymer was completely dissolved in 2.5ml ofdeionised water to yield a clear solution. One solution of each of thepairs described was poured into the other and then mixed thoroughly witha spatula. In some instances, such as for the poly(Tem)/(Spm) pair, thegelation was almost instantaneous, forming a thick, swollen mass thatincorporated all of the water from the system. If this was allowed tostand for a while, the gel could be seen to contract slightly, expellingsome of the water from the matrix. It should be noted at this stage,that gels were mixed on an equivalent weight basis rather than usingmolar proportions (of monomer feed or groups in polymer as analysed).

[0143] By talking the observations made in table 6 and plotting them interms of a ternary phase diagram, it can be seen that there are trendsvisible (FIG. 1). In polymer systems in which the hydrophobic componentis in high, the resulting polymers are water-insoluble and so cannotform a PIC from aqueous solution (although this may still be possiblefrom other solvent systems). In systems where the PC component is high,both the individual polymers and the resulting PIC remain water-soluble.When the correct balance of ionic/hydrophilic/hydrophobic is obtained, agel is formed as the polyions complex. This gel tends to be ‘stiffer’when the hydrophilicity is reduced and when the ionic content is higher.

[0144] Thus, a generalisation can be made for the formation of PICs inthis type of system (FIG. 2). The application in mind will determinewhat type of PIC will be required. For instance, if one requires theformation of a gel for filling an aneurysm, the properties required fromthat gel will be such that it remains in place once formed; henceforth,if its tendency is to flow, it will not be suitable.

EXAMPLE 3 Determination of the Gelation Properties of Polvion Complexes

[0145] When considering the ability of a mixture of two polyionsolutions to form a gel as described in FIG. 2, it is useful to be ableto quantify the observations made. In this instance, 20% (w/v) solutionsof the individual polymers were made, mixed together and allowed tosettle overnight. The resulting PICs were subected to a variable torqueis oscillation test (10-100 mN.m) using a TA Instruments CSL²-100rheometer fitted with 6 cm 2° cone at 37° C. From this, two parameterscould be measured, namely G′ the elasticity modulus and G″ the viscousmodulus. Table 7 summarises the measurements of these parameters for avariety of PIC mixtures, taken at 80 nN.m. The polyions are defined byreference to the monomer ratios used rather than from analysis ofionicgroups in the polymer.

[0146] Clearly, there a large spread in viscoelastic properties betweenthe different PICs formed. The values are in agreement with theobservations expressed in table 6 and reinforce FIGS. 1 & 2. Wherevalues of G′ and G″ are low, little gelation has occurred when solutionshave been mixed. Where these values are higher (ca.>10 Pa), a firm gelof has formed. When the value of G″ exceeds that of G′, the material hasmore viscous properties than elastic and it will tend to flow underapplied force rather than act elastically. Where G′ is greater than G″the opposite is true indicating a more elastic material with apropensity to withstand applied force. This is a useful measure of amaterials' potential behaviour in a particular application. Forinstance, if an aneurysm-filling material is considered, it would bedesirable to obtain a gel that will not wash out of the void under theinfluence of blood flow.

EXAMPLE 4 Gelation from solvent svstem.

[0147] A solubility study was performed on PC-PICs. They were found tobe soluble in ternary solvent mixtures of water, ethanol and NaCl. Theresults are shown in the ternary phase diagram FIG. 3.

EXAMPLE 5 Biological performance of PC PIC's

[0148] A solution of the PIC could then be used to produce reproduciblecoatings on PET that could be used for biological evaluation. Stripswere subjected to a double antibody fibrinogen assay (Fg) and microbicinchoninic acid protein assay (μ-BCA) in order to gain anappreciation of the extent of protein interaction with the materials.Table 8 summarises the results. Again the polyions are defined byreference to the ratios of monomers used.

[0149] From the data it can be seen that coatings of polyion complexesexhibit a lower degree of protein adsorption than the PET control strip.The comparison PIC made from mixing the homopolymers of Tem and Spm(5.3) is less effective at lowering the protein adsorption than thosePIC's that contain Mpc. This is consistent with the view that Mpcimproves the ‘biocompatibility’ of surfaces. TABLE 1 Preparative Detailsfor a Series of Polyions Reaction Reaction Initiator [Initiator] ScaleSolids Polymer Solvent Time (mins) Temp (° C.) Type (%) (g) (%) MpcTemD.I. Water 24 80 APS 1 30 15 MpcSpm D.I. Water 24 80 APS 1 30 15MpcBmaTem EtOH 24 70 AIBN 1 30 15 MpcBmaSpm EtOH 24 70 AIBN 1 30 15Mpc₄₀Bma₄₀Tem₂₀ THF/EtOH 18 70 AIBN 1 25 12.5 Mpc₄₀Bma₄₀Spm₂₀ TFE 24 70AIBN 1 25 12.5 Mpc₁₅Bma₃₅Tem₅₀ EtOH 18 70 AIBN 1 25 12.5 Mpc₁₅Bma₃₅Spm₅₀EtOH 18 70 AIBN 1 25 12.5 MpcTem₂ EtOH 24 60 AIBN 0.2 15 15 BmaSpm TFE40 60 AIBN 0.4 30 12.5 Mpc₁₅Tem₈₅ D.I. Water 24 80 APS 1 25 12.5Mpc₁₃Spm₈₅ D.I. Water 24 80 APS 1 25 12.5 Poly(Tem) D.I. Water 24 86 APS1 25 12.5 Poly(Spm) D.I. Water 24 86 APS 1 25 12.5

[0150] TABLE 2 Isolation Details for a Series of Polyions RedissolutionPrecipitation Yield Yield Polymer Solvents Solvent (g) (%) AppearanceComments MpcTem — — 15.8 53 Fine, white powder Isolated by freeze-dryingMpcSpm — — 27 90 Fine, white powder Isolated by freeze-drying MpcBmaTem120 ml DCM/5 ml MeOH 780 ml Acetone 22.6 75 Fine, white powder MpcBmaSpm120 ml DCM/5 ml MeOH 780 ml Acetone 16.9 56 Grey-white powderMpc₄₀Bma₄₀Tem₂₀ — 200 ml Acetone 13.8 55 Fine, white powderMpc₄₀Bma₄₀Spm₂₀ 140 ml DCM/80 ml TFE 1.21 Acetone 17.3 69 Fine, whitepowder Mpc₁₅Bma₃₅Tem₅₀ 120 ml DCM/5 ml MeOH 780 ml Acetone 16.3 65 Lumpywhite solid Mpc₁₅Bma₃₅Spm₅₀ 120 ml DCM/5 ml MeOH 780 ml Acetone 6.6 27Lumpy white solid Difficult to isolate (low Mw?) MpcTem₂ 48 ml DCM/4 mlMeOH 500 ml Acetone 13.5 95 White solid BmaSpm 50 ml DCM/20 ml TFE 1.51Acetone 26.8 89 Stringy solid Mpc₁₅Tem₈₅ — — ˜22.5 90 White solidEstimated yield by drying Mpc₁₅Spm₈₅ — — ˜22.5 90 White solid down asample of solution Poly(Tem) — — ˜22.5 90 White solid Estimated yield bydrying Poly(Spm) — — ˜22.5 90 White solid down a sample of solution

[0151] TABLE 3 Summary of ¹H NMR Data for a Series of Polyions. PolyionSolvent Description (Peak Positions) Comments Poly(Spm) D₂O 0.9-1.1 (3peaks, b); 1.95 (b); 2.15(s); 3.0 (triplet, —CH ₂—S—); 4.15(b) Asexpected for structure Poly(Tem) D₂O 0.9-1.2 (3 peaks, b); 2.05 (b); 3.3(s, N⁺(CH ₃)₃); 4.85 (m); 4.5 (b) As expected for structure Mpc₁₅Spm₈₅D₂O 0.8-1.2 (2 peaks, b); 1.9 (b); 2.15 (s); 3.0 (triplet, —CH ₂—S—);3.3 (s, Integration of (N⁺(Me)₃) vs. N⁺(CH ₃)₃); 3.7; 4.1-4.3 (2 peaks,b) —CH ₂—S gives expected formula Mpc₁₅Tem₈₅ CD₃OD 0.9-1.3 (3 peaks, b);2.0 (b); 3.26 + 3.31 (overlapping, N⁺(Me)₃ from Mpc Cannot integrate Mpcvs. Tem, peaks and Tem); 3.7-4.7 (6 peaks, overlapping, b) to close.MpcBmaSpm CD₃OD 0.8-1.3 (3 peaks, b); 1.45 (—CH ₂—CH₃); 1.65 (—O—CH₂—CH₂—); Integration of Mpc vs. Spm and 1.95; 2.15; 2.9 (triplet, —CH ₂—S—);3.3 (s, N⁺(CH ₃)₃); 3.7; elemental analysis suggests more like 3.9-4.4(3 peaks, b) ˜Mpc₂₅Bma₃₃Spm₄₀. Monomer contamination observed. MpcBmaTemCD₃OD 0.9-1.2 (2 peaks, b); 1.45 (—CH ₂—CH₃); 1.65 (—O—CH₂—CH ₂—);Cannot integrate Mpc vs. Tem, peaks 1.95 (b); 3.3 + 3.32 (overlapping,N⁺(Me)₃ from Mpc and Tem); to close. 3.7-4.7 (8 peaks overlapping, b)MpcSpm D₂O 0.9-1.1 (2 peaks, b); 1.9-2.2 (2 peaks, b); 2.95 (vaguetriplet, Integration shows 50:50 Mpc:Spm as —CH ₂—S—); 3.3 (s, N⁺(CH₃)₃); 3.7; 4.1-4.4 (3 peaks, b) expected. MpcTem D₂O 0.9-1.3 (2 peaks,b); 2.2 (b); 3.3 + 3.33 (overlapping, N⁺(Me)₃ from Mpc Cannot integrateMpc vs. Tem, peaks and Tem); 3.7; 3.9, 4.1-4.6 (3 peaks, b) to close.BmaSpm DMSO 0.7-1.0 (2 peaks, b); 1.35 (—CH ₂—CH₃); 1.55 (—O—CH₂—CH ₂—);Integration not possible as residual 1.85; 2.5 (—CH ₂—S— is masked byDMSO); 3.9 (b) undeuterated DMSO masks Spm. MpcTem₂ CD₃OD 1.0-1,3 (2peaks, b); 2.15 (b); 3.36 + 3.44 33 (overlapping, N⁺(Me)₃, from Cannotintegrate Mpc vs. Tem, peaks Mpc and Tem); 3.8-4.7 (7 peaks overlapping,b) to close. Mpc₄₀Bma₄₀Spm₄₀ CD₃OD 0.8-1.1 (3 peaks, b); 1.35 (—CH₂—CH₃); 1.55 (—O—CH₂—CH ₂—); Integration yields formula as expected. 1.8(b); 2.05 (b); 2.8 95 (triplet,—CH₂—S—); 3.24 (s, N⁺(CH ₃)₃); 3.7;3.9-4.3 (4 peaks, b), 4.6 Mpc₄₀ Bma₄₀Tem₄₀ CD₃OD 0.8-1.2 (2 peaks, b);1.35 (—CH ₂—CH₃); 1.55 (—O—CH₂—CH ₂—); Cannot integrate Mpc vs. Tem,peaks 2.1 (b); 3.24 + 3.28 (overlapping, N⁺(Me)₃ from Mpc and Tem);3.6-4.7 to close. (7 peaks overlapping, b)

[0152] TABLE 4 Selected P:N & P:S Ratios for the Confirmation of PolymerFormula (where applicable) Italics highlight cases where actual resultssignificantly differ from those of the feed ratio. Polycation % %Theoretical Actual (molar feed ratio) Mpc Tem Phosphorus Nitrogen P:NP:N % Mpc % Tem MpcTem 50 50 4.8 4.9 0.904 1.021 39 56.5 MpcBmaTem 33.333.3 4.28 3.9 0.904 0.911 29.7 33.6 Mpc₄₀Bma₄₀Tem₂₀ 40 20 4.28 1.840.678 0.43 30 12.7 Mpc₁₅Bma₃₅Tem₅₀ 15 50 2.17 3.91 1.957 1.802 13.9 46MpcTem₂ 33.3 66.7 3.2 5.05 1.356 1.578 24.4 77.5 Mpc₁₅Tem₈₅ 15 85 1.75.31 3.019 3.124 12.1 87.9 Polyanion % % Theoretical Actual (molar feedratio) Mpc Spm Phosphorus Sulphur P:S P:S % Mpc % Spm MpcTem 50 50 4.65.7 1.035 1.239 40.2 59.9 MpcBmaSpm 33.3 33.3 3.19 4.46 1.033 1.398 23.545.1 Mpc₁₅Bma₄₀Spm₂₀ 40 20 4.45 2.59 0.516 0.582 32.3 22.6Mpc₁₅Bma₃₅Spm₅₀ 15 50 1.98 6.61 3.444 3.338 13.9 48.5 Mpc₁₅Spm₈₅ 15 851.75 10.5 5.869 6 14.3 86.9

[0153] TABLE 5 Polymer Feed and Final Formulas Based on NMR andElemental Data Presented in TABLES 4 & 5. Where fee ratios differssignificantly from final ratio, the formula is shown in italics InherentViscosities obtained by Rheometry on 20% w/v Aqueous Solutions of thePolyions. Suggested Final Polymer Inherent Monomer Feed Formula FormulaViscosity (mPa · s) Poly(Tem) Poly(Tem) 40 MpcTem MpcTem 8.5 MpcBmaTemMpcBmaTem 10 Mpc₄₀Bma₄₀Tem₂₀ Mpc ₃₀Bma₅₅Tem₁₅ 18 Mpc₁₅Bma₃₅Tem₅₀Mpc₁₅Bma₃₅Tem₅₀ 14 MpcTem₂ MpcTem₃ 42 Mpc₁₅Tem₈₅ Mpc₁₅Tem₈₅ 71 Poly(Spm)Poly(Spm) 300 MpcSpm MpcSpm 130 MpcBmaSpm Mpc ₂₅Bma₃₅Spm₄₀ 11Mpc₄₀Bma₄₀Spm₂₀ Mpc₄₀Bma₄₀Spm₂₀ 6 Mpc₁₅Bma₃₅Spm₅₀ Mpc₁₅Bma₃₅Spm₅₀ 10BmaSpm BmaSpm 14 Mpc₁₅Spm₈₅ Mpc₁₅Spm₈₅ 250

[0154] TABLE 6 Some Observations Made upon Mixing Aqueous Solutions ofPolyions. Polycation Polyanion Gel Formed? Appearance Comments MpcTemMpcSpm No Viscous liquid Mpc₁₅Tem₈₅ Mpc₁₅Spm₈₅ Yes Thick gel OpaqueMpcTem SpmBma Yes Flowing gel Opaque MpcTem₂ SpmBma Yes Thick gelOpaque, expels water MpcBmaTem MpcBmaSpm Yes Flowing gel ClearMpc₁₅Bma₃₅Tem₅₀ Mpc₁₅Bma₃₅Spm₅₀ Yes Gel Clear Mpc₄₀Bma₄₀Tem₂₀Mpc₄₀Bma₄₀Spm₂₀ Yes Flowing gel Opaque MpcBmaTem MpcSpm No Viscousliquid MpcTem MpcBmaSpm No Viscous liquid Mpc₂₀Bma₆₀Tem₂₀Mpc₂₀Bma₆₀Spm₂₀ — — Polymers water-insoluble Poly(Tem) Poly(Spm) YesVery thick gel Opaque, expels water

[0155] TABLE 7 Viscoelastic Properties of Selected PIC gels PolycationPolyanion G′ (Pa) G″ (Pa) MpcTem BmaSpm 3.25 30 MpcTem BmaSpm 600 800MpcTem MpcSpm 0.15 3.5 MpcTem MpcBmaSpm 0.025 0.48 MpcBmaTem MpcSpm 0.34 MpcBmaTem MpcBmaSpm 50 45 Mpc₁₅Bma₃₅Tem₅₀ Mpc₁₅Bma₃₅Spm₅₀ 400 150Mpc₁₅Tem₈₅ Mpc₁₅Spm₈₅ 1500 1000 Mpc₄₀Bma₄₀Tem₂₀ Mpc₄₀Bma₄₀Spm₂₀ 85 125Poly(Tem) Poly(Spm) 9000 4500

[0156] TABLE 8 Estimation of Adsorbed Protein for PIC Coatings UsingFibrinogen (Fg) and bicinchoniic acid (μ-BCA) Assays (Uncoated PET stripcontrol) Bioevaluation % Reduction of No Polyion Complex Pair TestMethod Adsorbed Protein 5.1 MpcBmaTem + MpcBmaSpm Fg (n = 7) 77.8 5.2Mpc₁₅Bma₃₅Tem₅₀ + MPC₁₅Bma₃₅Spm₅₀ Fg (n = 7) 77.7 5.3 Poly(Tem) +Poly(Spm) Fg (n = 7) 47.1 5.1 MpcBmaTem + MpcBmaSpm μ-BCA (n = 5) 82.45.2 Mpc₁₅Bma₃₅Tem₅₀ + Mpc₁₅Bma₃₅Spm₅₀ μ-BCA (n = 4) 61.8 5.3 Poly(Tem) +Poly(Spm) μ-BCA (n = 3) 33.7

1. A polyion complex formed from a cationic polymer having an overallcationic charge and an anionic polymer having an overall anionic charge,in which the anionic polymer is obtainable by polymerising ethylenicallyunsaturated monomers including: a) 5 to 100 mole % anionic monomerhaving an anionic or anionisable group; b) 0 to 85 mole % zwitterionicmonomer having a zwitterionic group; and c) 0 to 80 mole % nonionicmonomer; and in which the cationic polymier is obtainable bypolymerising ethylenically unsaturated monomers including d) 5 to 100cationic monomer having a cationic or cationisable group; e) 0 to 85mole % zwitterionic monomer having a pendant zwitterionic group; and f)0 to 80 mole % non ionic monomer; in which the total units in thepolyion complex derivable from nonionic monomer c and f is in the range0 to 60 mole %, the total mole % of units in the polyion complexderivable from zwitterionic monomer is in the range 1 to 70 mole %, andthe ratio of moles of excess anionic or anionisable groups in theanionic polymer to the moles of excess cationic or cationisable groupsin the cationic polymer is in the range 1.5:1 to 1:1.5.
 2. A polyioncomplex formed from a cationic polymer having an overall cationic chargeand an anionic polymer having an overall anionic charge, in which theanionic polymer is water soluble and is obtainable by polymerisingmonomers including a) 5 to 100% anionic monomer having an anionic oranionisable group; and b) 0 to 85 mole % zwitterionic monomer havingpendant zwitterionic group; c) 0 to 60 mole % non ionic monomer; and inwhich the cationic polymer is water soluble and is obtainable bypolymerising ethylenically unsaturated monomers including d) 5 to 100mole % cationic monomer having a cationic or cationisable group; e) 0 to85 mole % zwitterionic monomer having a zwitterionic pendant group; f) 0to 60 mole % nonionic monomer; and in which the total moles of units inthe polyion complex derivable from zwitterionic monomers in the range 1to 70 mole %, and in which the ratio of total moles anionic groups inexcess of cationic groups in anionic polymer to total moles cationicgroups in excess of anionic groups of cationic polymer is in the range1.5:1 to 1:1.5.
 3. A polyion complex according to claim 1 or claim 2, inwhich the total of units derivable from nonionic monomer in the polyioncomplex is preferably at least 5 mole %.
 4. A polyion complex accordingto any preceding claim in which the anionic polymer is formed frommonomers substantially free of cationic monomer and the cationic polymeris formed of monomers substantially free of anionic monomer.
 5. Apolyion complex according to any preceding claim in which the ratio oftotal moles of anionic monomer used to form anionic polymer to totalmoles of cationic monomer used to form the cationic polymer is in therange 1.25:1 to 1:1.25, preferably in the range 1.1:1 to 1:1.1,preferably about 1:1.
 6. A polyion complex according to any precedingclaim which is water-insoluble and water-swellable.
 7. A polyion complexaccording to any preceding claim in which the zwitterionic monomerpreferably has the formula VI YBX   VI wherein B is a straight orbranched alkanediyl or alkanediyloxaalkanediyl oralkanediyloligo(oxaalkanediyl) chain optionally containing one or morefluorine atoms up to and including perfluorinated chains or, if X or Ycontains a terminal carbon atom bonded to B, a valence bond; X is thezwitterionic group; and Y is an ethylenically unsaturated polymerisablegroup selected from

CH₂═C(R)—CH₂—O—, CH₂═C(R)—CH₂OC(O)—, CH₂═C(R)OC(O)—, CH₂═C(R)—O—,CH₂═C(R)CH₂OC(O)N(R¹¹)—, R¹²OCCR═CRC(O)—O—, RCH═CHC(O)O—,RCH═C(COOR¹²)CH₂—C(O)—O—,

wherein: R is hydrogen or a C₁₋₄ alkyl group; R¹¹ is hydrogen or a C₁₋₄alkyl group or R¹¹ is —B—X where B and X are as defined above; and R¹²is hydrogen or a C₁₋₄ alkyl group or BX where B and X are as definedabove; A is —O— or —NR¹¹—; K is a group —(CH₂)_(p)OC(O)—,—(CH₂)_(p)C(O)O —, —(CH₂)_(p)OC(O)O—, —(CH₂)_(p)NR¹³—,—(CH)_(p)NR¹³C(O)—, —(CH)_(p)C(O)NR¹³—, —(CH₂)_(p)NR¹³C(O)O—,—(CH₂)_(p)OC(O)NR¹³—, —(CH)_(p)NR¹³C(O)NR¹³— (in which the groups R¹³are the sarne or different), —(CH)_(p)O—, —(CH)_(p)SO₃—, or, optionallyin combination with B, a valence bond and p is from 1 to 12 and R¹³ ishydrogen or a C₁₋₄ alkyl group.
 8. A polyion complex according to claim7 in which the zwitterion has the formula IV

in which the moieties X⁴ and X⁵, which are the same or different, are—O—, —S—, —NH— or a valence bond, preferably —O—, and W⁺ is a groupcomprising an ammonium, phosphonium or sulphonium cationic group and agroup linking the anionic and cationic moieties which is preferably aC₁₋₁₂-alkanediyl group.
 9. A polyion complex according to claim 8 inwhich W⁺ is a group of fonnula —W¹—N⁺R⁸ ₃, —W¹—P⁺R⁹ ₃, —W¹—S⁺R⁹ ₂ or—W¹—Het⁺ in which: W¹ is alkanediyl of 1 or more, preferably 2-6 carbonatoms optionally containing one or more ethylenically unsaturated doubleor triple bonds, disubstituted-aryl, alkylene aryl, aryl alkylene, oralkylene aryl alkylene, disubstituted cycloalkyl, alkylene cycloalkyl,cycloalkyl alkylene or alkylene cycloalkyl alkylene, which group W¹optionally contains one or more fluorine substituents and/or one or morefunctional groups; and either the groups R⁸ are the same or differentand each is hydrogen or alkyl of 1 to 4 carbon atoms, preferably methyl,or aryl, such as phenyl or two of the groups R⁸ together with thenitrogen atom to which they are attached form a heterocyclic ringcontaining from 5 to 7 atoms or the three groups R⁸ together with thenitrogen atom to which they are attached form a fused ring structurecontaining from 5 to 7 atoms in each ring, and optionally one or more ofthe groups R⁸ is substituted by a hydrophilic functional group, and thegroups R⁹ are the same or different and each is R⁸ or a group OR⁸, whereR⁸ is as defined above; or Het is an aromatic nitrogen-, phosphorus- orsulphur-, preferably nitrogen-, containing ring, for example pyridine.10. A polyion complex according to claim 8 or claim 9 in which thezwitterion is a group of formula V:

where the groups R¹⁰ are the same or different and each is hydrogen orC₁₋₄ alkyl, and m is from 1 to 4, preferably in which all groups R¹⁰ aremethyl.
 11. A polyion complex according to any preceding claim in whichthe anionic monomer and cationic monomer each have the formula VII Y¹B¹Q  VII in which Y¹ is an ethylenically unsaturated polymerisable groupselected from

CH₂═C(R¹⁵)—CH₂—O—, CH₂═C(R¹⁵)—CH₂ OC(O)—, CH₂═C(R¹⁵)OC(O)—, CH₂═C(R¹⁵—,CH₂—(R¹⁵)CH₂OC(O)N(R¹⁵)—, R¹⁷OOCCR¹⁵═CR¹⁵C(O—O—, R¹⁵CH═CHC(O)O—,R¹⁵CH═C(COOR¹⁷)CH₂—C(O)—O—,

wherein: R¹⁵ is hydrogen or a C₁₋₄ alkyl group; is R¹⁶ is hydrogen or aC₁₋₄ alkyl group or R¹⁶ is B¹Q where B¹ and Q are as defined below; R¹⁷is hydrogen or a C₁₋₄ alkyl group or B¹Q where B¹ and Q are as definedbelow; A¹ is —O— or —NR¹⁶—; K¹ is a group —(CH₂)_(r)OC(O)—,—(CH₂)_(r)C(O)O—, —(CH₂)_(r)OC(O)O—, —(CH₂)_(r)NR¹⁸—,—(CH₂)_(r)NR¹⁸C(O)—, —(CH₂)_(r)C(O)NR¹⁸—, —(CH₂)_(r)NR¹⁸C(O)O—,—(CH₂)_(r)NR¹⁸C(O)NR¹⁸—, —(CH₂)_(r)NR¹⁸C(O)NR¹⁸O— (in which the groupsR¹⁸ are the same or different), —(CH₂)_(r)O—, —(CH₂)_(r)SO₃—, or,optionally in combination with B¹, a valence bond and r is from 1 to 12and R¹⁸ is hydrogen or a C₁₋₄ alkyl group; B¹ is a straight or branchedalkanediyl, oxaalkylene, alkanediyloxaalkanediyl, oralkanediyloligo(oxaalkanediyl) chain optionally containing one or morefluorine atoms up to and including perfluorinated chains or, if Q or Y¹contains a terminal carbon atom bonded to B¹ a valence bond; and Q isthe ionic or ionisable group.
 12. A polyionic complex according to claim11 in which Q is a cationic group Q¹ which is a group N⁺R¹ ₃, P⁺R¹ ₃ orS⁺R¹ ₂ in which the groups R¹ are the same or different and are eachhydrogen, C₁₋₄-alkyl or aryl (preferably phenyl) or two or three of thegroups R⁸ together with the heteroatom to which they are attached from asaturated or unsaturated heterocyclic ring containing from 5 to 7 atoms,preferably each R⁸ being other than hydrogen, most preferably N⁺R¹ ₃ inwhich each R¹ is C₁₋₄-alkyl, preferably methyl.
 13. A polyion complexaccording to claim 1I in which Q is an anionic group Q² which isselected from carboxylate, carbonate, sulphonate, sulphate, phosphonateor phosphate, preferably a monovalent group, more preferably asulphonate group.
 14. A polyion complex according to any of claims 1 to10 in which the cationic monomer is diallyl dialkyl ammonium halide,preferably diallyl dimethyl ammonium chloride.
 15. A polyion complexaccording to any preceding claim in which the nonionic monomer has thegeneral formula VIII Y² R¹⁴ VIII in which Y² is an ethylenicallyunsaturated polymerisable group selected from

CH₂═C(R¹⁹-CH₂—O—, CH₂═C(R¹⁹)—CH₂OC(O)—, CH₂═C(R¹⁹)OC(O)—, CH₂═C(R¹⁹)—O—,CH₂═C(R¹⁹)CH₂OC(O)N(R²⁰)—, R²¹OOCCR¹⁹═CR¹⁹C(O)—O—, R¹⁹CH═CHC(O)O—,R¹⁹CH═C(COOR²¹)CH₂—C(O)—O—,

wherein: R¹⁹ is hydrogen or a C₁₋₄ alkyl group; R²⁰ is hydrogen or aC₁₋₄ alkyl group or R²⁰ is R1⁴; R²¹ is hydrogen or a C₁₋₄ alkyl group orR²¹ is R¹4; A² is —O— or NR²⁰—; K² is a group —(CH₂)₈OC(O)—,—(CH₂)₈C(O)O—, —(CH₂)₈OC(O)O—, —(CH₂)₈NR²²—, —(CH₂)₈NR²²C(O)—,—(CH),C(O)NR²²—, —(CH₂)₈NR²²C(O)O—, —(CH₂)₈OC(O)NR²²—,—(CH₂)₈NR²²C(O)NR²²— (in which the groups R²² are the same ordifferent), —(CH₂)₈O—, —(CH₂)₈SO₃—, or a valence bond and s is from 1 to12 and R²² is hydrogen or a C₁₋₄ alkyl group; and R¹⁴ is an opticallysubstituted C₁₋₂₄-alkyl or -alkenyl group, optional substituents beinghydroxyl groups; halogen atoms; alkoxy and oligo-alkoxy groups, in whichthe alkoxy groups have 1-6, preferably 2 or 3 carbon atoms; aryl groups,preferably optionally substituted phenyl groups (optional substituentsin a phenyl group being hydroxyl groups, halogen atoms or alkyl groups);acyl groups, especially C₁₋₆-alkanoyl groups; acyloxy groups, especiallyC₁₋₆-alkanoyloxy groups; or acylamino groups, especially C₁₋₆-alkanoylamino; in any ofwhich alkanoyl groups there maybe substituents selectedfrom halogen atoms and hydroxyl and alkoxyl groups.
 16. A polyioncomplex according to claim 15 in which R¹⁴ is C₄₋₁₈-unsubstituted alkyl.17. A polyion complex according to any preceding claim, when fullyswollen in water, has viscoelastic properties (determined using avariable torque oscillation test (80 rN.m) using a TA instrument CSL-100rheometer fitted with 6 cm 2° cone at 37° C.), G¹ (elasticity modulus)in the range I to 1000 and G″ (viscous modulus) in the range 1.5 to1000.
 18. A composition comprising a polyion complex according to anypreceding claim and a liquid absorbed in the polyion complex.
 19. Acomposition according to claim 18 in which the liquid is aqueous.
 20. Acomposition according to claim 19 in which the liquid is free of organicsolvent.
 21. A composition according to any of claims 18 to 20comprising an active agent selected from pharmaceutically active agentsand diagnostic agents.
 22. A method in which a solution of an anionicpolymer having an overall anionic charge and a cationic polymer havingan overall cationic charge together in a solvent system comprising afirst solvent and an inorganic salt which is water-soluble and formed ofmonovalent metal ions and monovalent counterions, in solution, is gelledby contact with water, whereby the ions of the inorganic salts becomedissociated from the polymer and extracted from the gel formed byelectrostatic attraction between polymer bound cationic groups andpolymer bound anionic groups, and is characterised in that at least oneof the cationic and anionic polymers comprises zwitterionic groups. 23.A method according to claim 22 in which said first solvent is an organicsolvent, preferably comprising a ketone or an alcohol.
 24. A methodaccording to claim 22 or claim 23 in which said solvent system comprisesfirst and second solvents which are miscible under the conditions of theprocess.
 25. A method according to claim 24 in which the said secondsolvent is water.
 26. A method according to claim 24 or claim 25 inwhich the ratio of first to second solvent is in the range 2:1-1:10,preferably 1:1-1:5.
 27. A method according to any of claims 22 to 26 inwhich the organic salt is a halide of an alkali metal, preferably achloride or bromide of sodium or potassium, more preferably sodiumchloride.
 28. A method according to any of claims 22 to 27 having any ofthe features of claims 1 to 16.